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LTC3548AIDDTRPBF Datasheet(PDF) 9 Page - Linear Technology |
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LTC3548AIDDTRPBF Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  LTC3548A 9 3548af Inductor Selection Although the inductor does not influence the operat- ing frequency, the inductor value has a direct effect on ripple current. The inductor ripple current ΔIL decreases with higher inductance and increases with higher VIN or VOUT: ΔIL = VOUT fO •L •1– VOUT VIN ⎛ ⎝⎜ ⎞ ⎠⎟ Accepting larger values of ΔIL allows the use of low inductances, but results in higher output voltage ripple, greater core losses, and lower output current capability. A reasonable starting point for setting ripple current is ΔIL = 0.3 • ILIM, where ILIM is the peak switch current limit. The largest ripple current ΔIL occurs at the maximum input voltage. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L ≥ VOUT fO • ΔIL •1– VOUT VIN(MAX) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this transition to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar elec- trical characteristics. The choice of which style inductor to use often depends more on the price vs size require- ments and any radiated field/EMI requirements than on what the LTC3548A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3548A applications. APPLICATIONS INFORMATION Figure 1. LTC3548A General Schematic VOUT2 RUN/SS2 VIN VIN = 2.5V TO 5.5V VOUT1 RUN/SS1 POR SW1 VFB1 GND VFB2 SW2 MODE/SYNC LTC3548A CIN R7 POWER-ON RESET C1 C2 L1 L2 R4 R2 R1 R3 COUT2 C4 C3 COUT1 3548A F01 PULSE SKIP* BURST* *MODE/SYNC = 0V: PULSE SKIP MODE/SYNC = VIN: Burst Mode OPERATION R6 R5 OPERATION |
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